Multiple sclerosis (MS) is a chronic inflammatory and demyelinating disease of the central nervous system (CNS), which is thought to be mediated by an erroneous attack of T cells on myelin autoantigens present in the central nervous system (CNS). Characterization of the T cell epitopes targeted in MS patients has remained technically challenging, and the functional characteristics of the neuroantigen-specific T cells, particularly in the CNS, have remained unresolved. We have obtained preliminary results showing vigorous T cell responses to MOG peptides in PBL of MS patients by cytokine ELISPOT assay. Based on these results, we want to examine the MOG- specific T cell response in individual patients longitudinally over the course of MS, and test the epitope specificity and functional avidity of these cells. To overcome the difficulties that arise when examining M0G responses directly ex vivo from the brain of MS patients, we propose to study T cell responses in the CNS of "humanized" HLA-DR2 and -DR4 transgenic mice. Our preliminary studies have indicated that T cell responses in HLA-DR transgenic mice are directed against similar MOG epitopes as T cell responses in MS patients with this HLA-DR haplotype. We will test the hypothesis that MOG-specific T cells undergo avidity maturation over the course of MS, and high-avidity T cells accumulate in the peripheral blood of patients prior to relapses or exacerbation of disease. Furthermore, we will test the function of MOG-specific T cells by studying the fine specificity and functional avidity of these cells in the CNS. We will test this hypothesis in the fol1owing aims: In Aim 1 we will examine the MOG epitope-specific T cell response over time in MS patients by cytokine ELISPOT. In Aim 2 we will test the avidity maturation of MOG-specific T cells over the course of disease in MS patients. In Aim 3 we will test the MOG-specific T cell repertoire in the CNS of HLA-DR2 and -DR4 transgenic mice over the course of EAE. In Aim 4 we will test the avidity maturation of MOG-specific T cells in the CNS and blood of the transgenic mice over the course of EAE. At the end of this project, we will have learned whether MOG-specific T cells undergo avidity maturation in MS patients and how this relates to relapses/remissions. The experiments in the HLA-DR transgenic mice will complement the human studies and define the specificity and function of MOG-reactive T cells in the CNS.